Sorry for the delay.

This is normal for dial-up, and in fact for any connection but with broadband the effect is not so dramatic.

The rise and fall of transfer speed is related to how many packets of data are being sent to your computer in any second. There are many factors that affect the speed, in particular how busy the internet servers are. Whilst packets are being sent to your dial-up, the servers carry out other tasks whilst it waits for your connection to respond saying, "I'm ready for more". The connection then has to wait for the servers to respond, and so on.

There is little you can do to change this.

Mark